cifs: Add support for mounting Windows 2008 DFS shares

Windows 2008 CIFS servers do not always return PATH_NOT_COVERED when
attempting to access a DFS share.  Therefore, when checking for remote
shares, unconditionally ask for a DFS referral for the UNC (w/out prepath)
before continuing with previous behavior of attempting to access the UNC +
prepath and checking for PATH_NOT_COVERED.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=31092

Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Sean Finney <seanius@seanius.net>
Signed-off-by: Pavel Shilovsky <piastry@etersoft.ru>
Signed-off-by: Steve French <sfrench@us.ibm.com>

1 files changed, 18 insertions, 0 deletions

diff --git a/fs/cifs/connect.c b/fs/cifs/connect.c
index b6c6a3629b0..c7ebeee38c6 100644
--- a/fs/cifs/connect.c
+++ b/fs/cifs/connect.c
@@ -2937,6 +2937,24 @@ try_mount_again:
                                (tcon->ses->server->maxBuf - MAX_CIFS_HDR_SIZE));
 
 remote_path_check:
+#ifdef CONFIG_CIFS_DFS_UPCALL
+        /*
+         * Perform an unconditional check for whether there are DFS
+         * referrals for this path without prefix, to provide support
+         * for DFS referrals from w2k8 servers which don't seem to respond
+         * with PATH_NOT_COVERED to requests that include the prefix.
+         * Chase the referral if found, otherwise continue normally.
+         */
+        if (referral_walks_count == 0) {
+                int refrc = expand_dfs_referral(xid, pSesInfo, volume_info,
+                                                cifs_sb, &mount_data, false);
+                if (!refrc) {
+                        referral_walks_count++;
+                        goto try_mount_again;
+                }
+        }
+#endif
+
         /* check if a whole path (including prepath) is not remote */
         if (!rc && tcon) {
                 /* build_path_to_root works only when we have a valid tcon */